Modern office printing devices, such as printers, photocopiers and facsimile machines, and the like, typically incorporate a media stack tray which is used to store and dispense media (such as paper) from a media stack to a printing head of the device. The dispensing of the media is usually performed using a dispensing device, such as a pick arm or a roller mechanism of some kind, which selects and dispenses media from the media stack tray.
The media stack tray is usually installed within the printing device in such a way that the media stack is not readily visible to a user. Accordingly, the status of the media stack tray, in terms of whether it is empty or loaded with media, is not able to be determined by visual inspection without first removing the media stack tray from the equipment.
To overcome the problem described above, it is common for media stack trays to include a mechanism which provides a visual indication of the status of the media stack tray.
A mechanism for sensing the status of a media stack tray which dispenses media may be of the type disclosed in U.S. Pat. No. 5,700,003. Here, the mechanism includes an actuator, which itself includes a paper contacting unit and an elastic unit. The elastic unit supplies a force to the actuator so that the paper contacting unit contacts the topmost paper sheet of a stack of paper in the media stack tray and applies a downward force onto the same so that the actuator moves downwardly after the topmost sheet is dispensed.
The actuator is connected to a variable resistor so that as the actuator moves downwardly the resistance of the resistor effectively varies according to the height of the paper stack within the media stack tray. The resistance of the resistor is used to determine the status of the media stack tray.
Unfortunately, in mechanisms of the type disclosed in U.S. Pat. No. 5,700,003, the paper contacting unit contacts each sheet which is dispensed from the media stack tray. Such contact may interfere with, or indeed damage (for example, by scratching or marking), the paper which is being dispensed from the top of the paper stack. Where the contact is of a type which exerts a force onto the sheet which is being dispensed, the interference may cause the sheet to be skewed during a dispensing cycle.
U.S. Pat. No. 6,206,362 describes another media stack status indicator. That indicator also includes a paper contacting unit (in the form of a probe member) which rests on the top of a media stack in a media stack tray (such as a tray). Thus, this arrangement shares similar problems to those described in relation to the mechanism disclosed in U.S. Pat. No. 5,700,003.
Another type of mechanism for sensing the status of a media stack tray is disclosed in U.S. Pat. No. 5,236,348. Here, a pivotally mounted indicator is positioned within a housing of the media stack tray. The indicator includes a portion which contacts with a surface of a biased sheet stack platform which is itself movable according to the weight of the media stack on the stack platform. Unfortunately, arrangements of this type have difficulty differentiating between there being no sheets and a few sheets on the stack platform, particularly for media having a light weight.